1. Field of the Invention
This invention relates to valve systems that employ a moving valve stem surrounded by multiple packing sets that are compressed by a moving packing follower.
2. Description of the Prior Art
For sake of clarity of comparison and brevity only, and not by way of limitation, the prior art valves and the valves that employ this invention will be described as though the movable stem was disposed vertically. The valves of this invention, like those of the prior art, can be employed in any spatial orientation from a vertical stem (up or down) to a horizontal stem, and any angle there between.
This invention also applies to moving stem valves in general. For example, this invention applies to an elongate stem that 1) is moved directly along its longitudinal axis to open and close the fluid flow passage of the valve body, 2) is rotated about its longitudinal axis to effect movement along that longitudinal axis to open and close the fluid flow passage of the valve body, e.g., gate type valves, and 3) is rotated about its longitudinal axis to open and close the fluid flow passage of the valve body without movement of the stem along its longitudinal axis, e.g., butterfly, ball, or plug type valves. The various structures for and operations of the foregoing types of stem valves are well known in the art and further description is not necessary to inform the art.
Again, for sake of clarity of comparison and brevity only, and not by way of limitation, the prior art valves and the valves that employ this invention will be disclosed in detail hereinafter in respect of valves whose stem is moved directly along the longitudinal axis of the stem to open and close the fluid flow passage of the valve body
Heretofore, valve assemblies using a moving stem carrying a control member to close and open a fluid flow passage in a valve body have employed a packing set surrounding the movable (along the long axis and/or around the long axis) stem with a packing follower pressed onto the top of the packing set to apply a sealing compressive force to the packing and stem. Such assemblies are reliable, but not sufficiently reliable to satisfy the requirements for valves that are to be used in hazardous waste service.
Hazardous waste service requires double containment of the packing, i.e., more than one packing set around the stem, leak detection, and controlled bleed off of process fluid that manages to by-pass the lower most (primary) packing set.
Employing at least one additional packing set above the primary packing set does not provide satisfactory double containment service for hazardous waste purposes. This is so because transmitting the packing compression load by way of a movable packing follower at the top of a packing stack containing multiple packing sets does not uniformly transmit the sealing compressive loading force from that packing follower through the upper most packing set to the lower most packing set. Put another way, the friction in the stack of multiple (two or more) packing sets has been found greatly to reduce the compressive load going down the stacked packing sets. This results in one or more of the lower packing sets, particularly the primary packing set, receiving insufficient sealing compression force from the outset.
Accordingly, it is desirable that there be available a packing system that employs a packing stack of two or more packing sets that at all times from the outset provides adequate sealing compression force to all packing sets in the stack, particularly the primary packing set.